You only have to make the mistake once: you go on a once in a lifetime trip, shoot rolls upon rolls of film that will make for beautiful portfolio work, Christmas cards, memory collections, and gifts. You get home and develop the film rolls, excited to see the trip in visual reproduction. But the pictures are absent, streaked, or foggy. How do you avoid such a mishap?

Protecting Your Film from the Obvious Elements
Heat and moisture are not the best of friends to film. When packing it in a carry-on leave the rolls in their original plastic canisters and outer boxes.

Put them in a plastic sealable baggie or a special photographic film pouch designed to protect film.

And pack the film with silica gel packets, which will protect it from humidity (as the plastic baggies protect it from sand, dirt, dust, and of course, water).

Protecting Your Film from Airport Scanner Damage
Both exposed and unexposed photographic film can suffer damage by the FAA (Federal Aviation Administration) regulated and enforced explosive-detection technology, high tech x-ray scanning machines at all major airports, used to scan checked luggage. (And according to the photographic professionals, higher shutter speed numbered film is especially vulnerable.)

These scanning machines either operate like hospital x-ray scanners, sending first a low dose of rays which are immediately followed by higher dosages for a double scan of luggage, or work at higher dose levels to scrutinize more closely parts of luggage not revealed by the first scanning operation.

While the first methods used can damage film, the second level methods are sure to destroy it. Unprocessed film that is scanned directly will show a foggy film over every picture.

And, evidently, the faster the film speed, the denser the fog. So with the two types of x-ray systems, all film (35mm, 120/220 speeds, 400-ft film rolls, APS--Advanced Photo System, ECN in cans, and sheet film) is at risk.
So first, you must avoid carrying film and cameras containing film in your luggage. Instead, pack it in your carry-on, in an easily reached location in the bag.

Though it has been said that the carry-on baggage scanners emit a low enough dose of x-radiation that “most” films will not incur damage, it is still better to play on the safe side, just in case.

Get to the airport early enough so that when you approach the inspection gates, you can insist that the film be hand-inspected. In the U.S., this is an FAA-protected right.

If airport security personnel require your carry-on be checked with the luggage, be sure to remove the film and any cameras with unprocessed film in them. Hand carry the film, which you can have in a special carrying case designed for shielding film, through the security walk-through gates.

The walk-through archway and the hand wand security uses to scan your person will not affect film, usually. If possible, though, ask to hand the bag to the airport personnel (though they will likely insist you do so, anyway).

It used to be (in the low/no-risk days of travel) that people would recommend lead-lined bags for film. But the power to block the high-intensity radiation depends on the quality of the lead bag (so you should ask for confirmation of effectiveness from the retailer or manufacturer). The use of a lead-lined carry-on will also often cause suspicion (trigger investigations that include scanning the item until security can identify it, which ultimately destroys the film, as well).

Protecting Your Film by Shipping it Home
It might be important enough that before you enter an airport for home, you mail yourself the unprocessed rolls. Just be sure to check with the postal service technicians about what x-ray processes and procedures they use, as their security techniques are advancing along with those of airport measures, in order to protect people.

Protecting Your Film by Printing in Advance
One way to avoid the unwanted damage to undeveloped film rolls is to develop them while you are on your trip. Provided you leave ample time for the processing of the negatives and publishing of the prints, you will have pictures instead of rolls of film to be concerned about. Pictures are, of course, less heat, weather, and x-ray sensitive and can be stored in your luggage without worry.

At the same time, digital camera images are not impacted in any way by airport scanners, so this might be the way to go when traveling. Either digital cameras, then, or buying film after you arrive at your destination and developing it before you depart might be the best measures you can take.